Alloy capacitor porous anodes

ABSTRACT

Alloys of intermetallic compounds and solid solutions are prepared as powders, compacted, and sintered at high temperatures to form porous, single phase alloys; the porous alloys are then surface-oxidized such as by anodization to form a mixed metal oxide film on the surfaces of the porous structure. This results in a mixed metal oxide surface film having good dielectric properties. The placement of conductive material in the void space of the porous anode results in the formation of a capacitor structure of superior properties. The general formula for the sintered alloys is AB where A and B are metals in the form of solid solutions or intermetallic compound alloys. Of specific interest are those 1:1 compounds which form ABO3 oxides. Preferred materials include Yal, CoTi and NiTi alloys. These AB sintered alloys are oxidized or anodized to form surface films of ABO3 and AxByOz.

United States Patent Kirkpatrick et al. [45] May 23, 1972 [54] ALLOYCAPACITOR POROUS ANODES 3,166,693 1/1965 Haring et al. ..3l7/230 [72]Inventors: Milton E. Kirkpatrick, Palos Verdes Prima ry Exammer.lames D.Kallam Pe mnsula Ralph Mendelson West Attorney-Da.nie1 T. Anderson,James V. Tura and Alan D. mmster, both of Calif. Akers [73] Assignee:TRW Inc., Redondo Beach, Calif. [57] CT 22 Filed: June 1, 1970 Alloys ofmtermetalhc compounds and solid solutlons are [21] Appl. No.: 42,087prepared as powders, compacted, and sintered at high temperatures toform porous, single phase alloys; the porous alloys are thensurface-oxidized such as by anodization to form a 2% F' mixed metaloxide film on the surfaces of the porous structure. 1 1 233 This resultsin a mixed metal oxide surface film having good [581 d 0 dielectricproperties. The placement of conductive material in the void space ofthe porous anode results in the formation of [56] References cued acapacitor structure of superior properties.

UNITED STATES PATENTS v The general formula for the sintered alloys isAB where A and B are metals in the form of solid solutions orintermetallic 1,682,846 9/1928 Kremers 317/233 compound am) of specificinterest are those m 1,709,427 4/1929 Bush ..3 /230 pounds which formA303 oxides Preferred materials include l'924'606 8/1933 Hammond 317/231Yal, CoTi and NiTi alloys. These AB sintered alloys are oxi g l i idizedor anodized to form surface films of ABC; and A,B,,O,. ray et a2,504,178 4/1950 Burnham et al. ..317/230 16 Claims, 7 Drawing FiguresYAL w Patented Mn 23, 1972 3,665,260

4 Shanta-Sheet l Fig.2

YAL IOOX Fig! Milton E. Kirkpatrick 'Dh A. Mendelson INVENTORS ATTORNEYPatnted M0123, 1972 3,665,260

4 shun-sh: 2

YAL ANODE IOX Fig.3

5OO M (N03) 2 IMPREGNATED 200 X Milton E. Kirkpatrick 4 Ralph A,Mendelson INVENTORS ATTORNEY Patntd May 23, 1972 v4 shank-shut 5 Ti. NiIOOX DARKFIELD Co IOOX Kirkpatrick Hon E nv O B e x "0 9T N m h m G .R gF ATTORNEY Patenteu May 23, 1972 3,665,260

4 Santa-Shout 4- Bakelite Alloy Matrix Electron Micrograph of ExposedOxide Oxide Layer Layer from a Nickel-Titanium Alloy Capacitor(Magnification 20,000 X; l J. fiducial marks) ,/gg3g Electrode Fig.7

MilfOfl E. Kirkpatrick Ralph A. Mendelson INVENTORS ALLOY CAPACITORPOROUS ANODES BACKGROUND OF THE INVENTION This invention relates to newand improved porous anodes for capacitors and more specifically tocapacitors of the ABO, and A,.B,,O, type which are composed of porous,surface-oxidized particles, such as those of YAl, CoTi and NiTi.

The use of tantalum capacitors including the porous tantalum types arewell known in the electronic field. While their capacitance on an anodevolume basis or on an anode weight basis is excellent, tantalum is quiteexpensive.

It is known that several of the group of mixed metal oxides involvingthe general composition ABC, exhibit extremely high dielectricproperties. An example of these compound types is barium titanate (BaTiOwhich has been used as ceramic capacitors for many years because of itshigh dielectric constant.

One of the main problems which have prevented the development of BaTiporous alloy anodes for porous sintered capacitors is the fact that thetwo metallic elements have widely different melting temperatures. As aresult, almost insurmountable difficulties are encountered in theformation of these alloys. Porous metallic alloys of other pure metalsdo not provide significant increases in dielectric properties.

It is known to sinter together particles of BaTiO to form ceramiccapacitors; however, they do not provide the high surface area of porousstructures. Therefore, their capacitance is based on the externalsurface area rather than the internal surface area of a sponge-likeporous structure such as the porous slug type anodes used in tantalumslug capacitors.

It is, therefore, an object of the invention to provide new and improvedcapacitor materials through the use of alloy materials.

Another object of this invention is to provide porous sintered anodes ofalloys of the type AB which, when oxidized, form A80 and A,B,,O, types.

Another object is to provide porous sintered anodes for capacitors ofcompositions comprising YA], NiTi and CoTi which can be oxidized oranodized to form high quality dielectric films.

Another object is to provide new compositions of matter comprising:porous, sintered alloys of metals A and B which form ABO and A,B.,O,upon surface oxidation.

Another object is to provide new compositions of matter comprising:porous, sintered, surface-oxidized particles of YA], CoTi and NiTi.

According to the invention, a solid solution or an intermetallic ABalloy such as YAl, NiTi or CoTi is first formed and then converted to afine powder, such as by crushing. The powder is then compacted into asuitable anode shape and sintered at elevated temperatures; this willproduce a porous sintered anode having an interconnected structure. Thecompacted porous structure is then surface oxidized; this may be carriedout in air or by permeating the porous structure with a suitableelectrolyte and then anodizing.

In contrast to the above-mentioned BaTiO ceramic capacitors, the porousinterconnected structures of the present invention provide a large,internal surface area which in turn yields a much larger capacitance.

The invention may be more readily understood from the description tofollow and from the diagrams in which:

FIGS. 1, 2, 4, 5 and 6 are photomicrographs of alloy microstructures invarious stages of production;

FIG. 3 is an electron micrograph showing an anode section which ismounted in a base; and

FIG. 7 is an electron micrograph of a NiTi oxide film coating on ananodized capacitor.

The first stage in the production of a YAl capacitor is the productionof a 1:1 YAl alloy which exists as an intermetallic compound. The weightpercent of the metals employed in order to obtain the 1:1 alloy is 24.0W/O Al and 76.0 W/O Y.

The Al and Y in the form of powders were are melted to achieve goodmixing and consolidation of the two elements:

X-ray analysis established that alloying had, in fact, taken place. FIG.1 shows the photomicrograph of the Al/Y compound and indicates that asingle phase alloy had been formed. The alloy remained stable under allfabrication procedures including cutting, grinding, polishing, andetching to bring out the microstructure.

It should be emphasized at this point that alloys of intermetalliccompounds of the 1:1 atomic composition can, in many cases, exist over arange of compositions and that this compositional variation may resultin alloys somewhat different in atomic ratio than the 1:1 composition,but having the same crystal structure as the stoichiometric l:lcompound. Such compounds, even though they vary over a limited range incomposition, will produce similar results when oxidized, with regard totheir application as capacitor anodes. This compositional variation isdue in part to both atomic and thermodynamic considerations.Additionally, compositional variations may result due to manufacturingprocess variables which can occur during the melting and solidificationof the alloy. Thus, a range of alloy compositions around the intendedstoichiometric composition will result in similar capacitor properties.

After the YAl ingot was formed, it was then crushed to a powder. This isa relatively easy procedure because the YAl intennetallic compound isextremely brittle. The crushing produced powders which could beclassified into four distinct size ranges: 500, -400, to '+500, 325 to'+400, and +325 mesh. The +325 mesh fraction was recrushed several timesin order to produce a powder which could be classified into the firstthree size ranges above mentioned.

The next stage of capacitor production involves compaction and sinteringof the powders into porous anodes. Various compaction procedures may beemployed; however, the one used is described in US. Pat. No. 3,496,425.Electric terminals in the form of metallic lead wires were applied byincorporating into the anodes during compaction.

FIG. 2 shows a compacted YAl powder from a 325 mesh fraction.

Following compaction and sintering, a yttrium aluminum oxide coating isformed on the surfaces of the YAl alloy particles. The oxide film can beproduced by impregnating the porous anode with 0.lN KOH as theelectrolyte, and then anodizing using a current of 40 ma at volts. Othermethods of forming oxide coatings will be disclosed herein. FIG. 3 showsthe anode.

The final step in the manufacture of the capacitor is the introductionof a counter electrode. This may be accomplished by coating theinterconnected, porous AB oxide film with a conductive film. One methodof applying the coating of conductive film is by dipping the anode in asolution of Mn(NOg)9 a preferred specific gravity of the MN(NO is 1.41.The dipping will cause the porous anode structure to be impregnated withthe solution of Mn(NO The anode structure is then baked to convert theMn(NO,'.)- solution to a MnO, conductive film coating over the AB oxidefilm. The dipping and baking procedure is preferably repeated until theentire porous space is filled with the conductive MnO,.

The microstructure of the completed capacitor is shown in FIG. 4.

The MnO, appears as the grey area. The voids or porous area are theblack portions and the white areas are the YAl alloy particles.

Tables I and II show the processing conditions and capacitance valuesfor anodes of YAl particles coated with yttrium aluminum oxide; atantalum anode formed in H PO. using a formation voltage of about 100volts is also shown by comparison. Measurements of the capacitance weremade with the anodes in the electrolyte using a General Radio l6l5Abridge at 100 I-I,.

TABLE I Compaction Sintering Oxide Film Pressure Temperature FormationAnode Powder (TSl) C.) & Voltage Times YAl (l) -500 mesh 2.5 1000, 1 hr.100 YAl (2) 325 mesh 2.5 800, min. 98 YAl (3) 325 mesh 1050, 1 hr. [02YAl (4) 325 mesh 10 1050, 1 hr, 101

hydrided/ dchydrided YAl (5) 325 mesh 3.5 X110, 1 hr. 102 10 325 mesh AlYAl (6) -325 mesh +2 3.5 1110, 1 hr. I02

Nopco wax 22 TABLE II Capacitance 2 Anode Volume Anode Weight AnodeCapacitance (uf) f/cm?) (pf/g.)

Ta (size c) 83.2 269 35.8 YAl (1) 52.2 440 213.0 25 YAl (2) 21.4 23574.7 YAl (3) 44.8 427 168.0 YA] (4) 41.1 367 154.0 YAl (5) 21.0 184 76.6YAl (6) 20.5 183 71.6

It will be observed from Tables I and II that the YAl anodes 2 and 3,which were formed from the same sized mesh powder but at widelydifferent pressures and sintering temperatures, exhibit markedlydiffering capacitances. Thus, the process permits capacitors having awide range of capacitance values to be produced. It will also beobserved that the capacitance of the YAl oxide coated anodes in terms ofanode weight is markedly superior to the tantalum anodes.

From the tables, it will appear that good results are obtained using asmall particle size (500 mesh).

Alloys of NiTi and CoTi were next fabricated using a Materials ResearchCorp. Series V-4 electron beam vacuum melting module. In both cases,55.0 W/O Co and 55.0 W/O Ni were employed with the balance being 45.0W/O Ti; this produced the 1:1 intermetallic compound in both cases.X-ray diffraction analysis of the alloy buttons indicated that the 1:1compound was indeed formed and metallographic analysis were made on thealloys in order to determine the microstructure. FIGS. 5 and 6 arephotomicrographs at 100 magnification showing the 1:1 NiTi and CoTimicrostructures respectively.

Following production of the 1:1 alloys into ingots, they may be powderedand compressed using the same techniques as disclosed for the YAl powderpreparation.

The preferred method for producing the oxide film on the alloys involvesanodization. For this purpose, five different electrolyte compositionswere employed for the NiTi and CoTi alloys; viz., H 80 H -,PO,,, er o, HSO. plus oxalic acid, and a concentrated l5 volume percent) H 80,solution. Utilizing Pourbaix diagrams, a pH of 2 appeared most suitablefor maintaining the nickel and cobaltin their oxidation states duringanodization. The best anodization of the NiTi compound was obtained withthe H 80, solution while the CoTi alloy gave the best anodized filmswith H PO lt was determined that where the pH exceeded 2 (e.g., 3 and5), the anode was etched rather than anodized, while a pH of 1 or lesscaused rapid metal dissolution which disrupted the film forming process.In the case of the YAl compound, very good films were also obtained byutilizing either H 80 or l-l PO at a pH of 2.

Using a forming voltage of volts and current densities of about 100 or200 milliamps per square centimeter, satisfactory thick films wereformed on the alloys. Current densities less than 200 ma/cm producedvery little if any film on the anode, but rather etched the surface.Film uniformity was achieved by employing a DC voltage with a maximum ACripple of less than 1 percent. Current control is achieved by insertingdummy anodes under test. 304 ELC stainless steel was employed as thecathode and dummy anode.

The bath temperature was approximately 20 F. and it does not appear thattemperature variation is an important variable of the anodizationprocess.

X-ray powder diffraction patterns were taken of the oxide film formed byanodization; these films had been removed from the samples of NiTi andCoTi. The patterns indicated that in all cases the oxide films wereamorphous in nature. In the case of the YAl compound, an X-raydiffraction pattern taken of the H PO anodized film showedcrystallinity.

To determine the capacitance properties of a NiTi capaci' tor, ananodized sample was coated with copper. and capacitance tests were madeusing a Boonton Electronics Corp. Model 74D Capacitance Bridge. The testfrequency was 0 100 Kl-l, with a 10 mV peak to peak test signal. For acapacitor area of 0.03 l 2 square inches and an anodization voltage of35 volts, the capacitance was 42,300 pF; conductance 1,750uhos; andstorage factor 15.2.

In order to accurately measure the anodization thickness and structure,the capacitor was ground to expose the counter electrode/anodizedlayer/base alloy and the exposed portion was then polished. An electronmicrograph was made as shown in FIG. 7. The copper electrode, oxidelayer region and the base alloy of nickel titanium can be seen in themicrograph. It will be apparent from the electron micrograph that anoxidation thickness of 0.7 micron was obtained, and this is in agreementwith that of photomicrographs which also were taken.

Based on the film thickness of 0.7 micron .from the electron micrograph,the dielectric constant of the anodized film on the 1:1 NiTi alloy isapproximately 160. Tantalum oxide has a dielectric constant ofapproximately 25.

As an example of alternate methods for oxidation of the aforementionedAB alloys, oxide films may be grown on sintered, porous particles ofCoTi and NiTi in air at about 867 C. for 24 hours. For example, oxidefilms were grown on polished samples of YAl, CoTi and NiTi in air atabout 867 C. for 24 hours. X-ray diffraction analysis of both the NiTiand CoTi oxide films showed the presence of a mixed metal oxidestructure.

Capacitance measurements were made of the CoTi-thermal oxide-Au formedin the argon-air atmosphere. Assuming a film thickness of approximately1,000 A as expected from the blue interference color, the dielectricconstant is well above 100.

In addition to forming oxides by exposure to air, other techniques forgrowing the oxide film are available. For example, by exposing thickfilms of NiTi and CoTi to an inert gas containing oxygen such as percentargon and 10 percent oxygen, at C. for 1 hour, adherent blue coloredfilms (due to interference effects) were formed. Gold electrodes wereapplied to the oxide film by evaporation techniques. Of course, use ofvacuum deposited counter electrodes may also be employed from thestandpoint of either fabrication and capacitance measurement. Theseelectrodes include platinum, copper, etc.

The air oxidation and argon-air oxidation techniques may be applied tothe porous capacitors in addition to the thin films as disclosed above.

It will be recognized that many interrelated parameters are possiblewithin the scope of this invention. These parameters include: particlesize, the chemical, electrical and metallurgical properties of aparticular AB compound and AB oxide, the powdering process, compactionprocedures such as pressure and time, sintering temperatures and times,anodization or other oxide film forming procedures, desired performancecharacteristics such as: capacitance and leakage properties, operatingrange, reliability, etc.

In the claims:

1. A process for producing porous alloy anodes comprising:

a. forming intermetallic compounds of metals selected from the classconsisting of: Y, A1, Co, Ni and Ti, said compounds forming dielectricoxide films upon oxidation;

bi powdering the said compounds;

c. compacting and sintering the said powder to form a porous,interconnected structure;

d. surface oxidizing the porous interconnecting alloy; and

e. applying an electrical terminal to said structure to form an anode.

2. A process for producing porous alloy capacitors which comprises:

a. forming intermetallic compounds of metals selected from the classconsisting of: Y, A1, Co, Ni and Ti, said compounds forming dielectricoxide films upon oxidation; b. powdering the said compounds; c.compacting and sintering the said powder to form a porous,interconnected structure; d. applying electrical terminals to form ananode; e. surface oxidizing the porous interconnecting alloy; and f.forming a counter electrode on the oxidized surface. 3. The process ofclaim 2 in which the atomic ratio of the AB metals in the said compoundis 1:1.

4. The process of claim 2 in which the said compound is YAl.

5. The process of claim 2 in which the said compound is NiTi.

6. The process of claim 2 in which the said compound is CoTi.

7. The process of claim 2 in which the said oxidation is by anodization.

8. The process of claim 2 in which the said oxidation is by airoxidation.

9. A porous alloy anode for an electric capacitor comprising:

a. a powder-compacted, sintered mass defining a porous, in-

terconnected structure;

b. said structure comprising intermetallic compounds of metals selectedfrom the class consisting of: Y, Al, Co, Ni and Ti, said compoundsforming dielectric oxide films upon oxidation; and c. an electricalterminal electrically interconnected with said structure. 10. A porousalloy capacitor comprising: a. a powder-compacted, sintered massdefining a porous, in-

terconnected structure; b. said structure comprising intermetalliccompounds of metals selected from the class consisting of: Y, Al, Co, Niand Ti, said compounds forming dielectric oxide films upon oxidation; 0.an electrical terminal electrically interconnected with said structure;d. the porous interconnections being coated with the oxide film of saidcompound; and e. a counter electrode electrically connected to saidoxide film. 11. The capacitor of claim 10 in the ratio of the AB metalsin the said compound is 1:1.

12. The capacitor of claim 10 in which the said compound is YAl.

13. The capacitor of claim 10 in which the said compound is NiTi.

14. The capacitor of claim 10in which the said compound is CoTi.

15. The capacitor of claim 10 in which the said oxide film is formed byanodization.

16. The capacitor of claim 10 in which the said oxide film is formed byair oxidation.

2. A process for producing porous alloy capacitors which comprises: a.forming intermetallic compounds of metals selected from the classconsisting of: Y, Al, Co, Ni and Ti, said compounds forming dielectricoxide films upon oxidation; b. powdering the said compounds; c.compacting and sintering the said powder to form a porous,interconnected structure; d. applying electrical terminals to form ananode; e. surface oxidizing the porous interconnecting alloy; and f.forming a counter electrode on the oxidized surface.
 3. The process ofclaim 2 in which the atomic ratio of the AB metals in the said compoundis 1:1.
 4. The process of claim 2 in which the said compound is YAl. 5.The process of claim 2 in which the said compound is NiTi.
 6. Theprocess of claim 2 in which the said compound is CoTi.
 7. The process ofclaim 2 in which the said oxidation is by anodization.
 8. The process ofclaim 2 in which the said oxidation is by air oxidation.
 9. A porousalloy anode for an electric capacitor comprising: a. a powder-compacted,sintered mass defining a porous, interconnected structure; b. saidstructure comprising intermetallic compounds of metals selected from theclass consisting of: Y, Al, Co, Ni and Ti, said compounds formingdielectric oxide films upon oxidation; and c. an electrical terminalelectrically interconnected with said structure.
 10. A porous alloycapacitor comprising: a. a powder-compacted, sintered mass defining aporous, interconnected structure; b. said structure comprisingintermetallic compounds of metals selected from the class consisting of:Y, Al, Co, Ni and Ti, said compounds forming dielectric oxide films uponoxidation; c. an electrical terminal electrically interconnected withsaid structure; d. the porous interconnections being coated with theoxide film of said compound; and e. a counter electrode electricallyconnected to said oxide film.
 11. The capacitor of claim 10 in the ratioof the AB metals in the said compound is 1:1.
 12. The capacitor of claim10 in which the said compound is YAl.
 13. The capacitor of claim 10 inwhich the said compound is NiTi.
 14. The capacitor of claim 10 in whichthe said compound is CoTi.
 15. The capacitor of claim 10 in which thesaid oxide film is formed by anodization.
 16. The capacitor of claim 10in which the said oxide film is formed by air oxidation.